An electric machine with a rotor which is rotatably mounted in a housing with a rotor shaft which extends beyond the housing, a plurality of electromagnet components which are statically disposed in the housing at uniform angular spacings and spaced from the axis of rotation of the rotor, each with a coil core bearing a coil winding consisting of one or more conductors and with permanent magnets which are disposed at uniform angular spacings and are non-rotatably retained in or on the rotor, these permanent magnets each having a pole face aligned opposite the end faces of the coil cores and each having a polarity which is successively reversed in the peripheral direction, wherein the coil cores of the electromagnet components are disposed parallel to the axis of rotation of the rotor shaft in the interior of the housing in such a way that their opposing end faces each lie in two planes which are spaced from one another and extend at right angles to the axis of rotation of the rotor shaft and the ends of the electric conductors which form the coil winding of the individual electromagnet components are interconnected via an electric or electronic control device to form at least two pairs of electrical connections and the rotor has at least two outer armature discs which extend radially to before the end faces of the coil cores and in which the permanent magnets are retained with their pole faces aligned with the respective associated end faces of the coil cores, the radially inner region of the said armature discs being provided with holes.
Such machines which can be connected to electric current sources are used for example as wheel hub motors for vehicle drives because of the favourable power-weight ratio. In this case these motors are designed so that the permanent magnets are retained in a disc, which is made from non-magnetic material and is non-rotatably connected to the rotor shaft, in such a way that their pole faces open freely on both sides into the flat sides of the disc-shaped rotor armature, the magnets being disposed so that in the peripheral direction successive pole faces on the same side of the armature have alternating polarities. The electromagnet components in the form of coils wound on metal cores are disposed on the inner faces of the housing cover or radial housing walls facing the disc-shaped rotor armature at a distance corresponding to the average radial distance of the permanent magnets from the axis of rotation of the rotor shaft. The control of these coils is achieved preferably by an electronic control device which generates a magnetic rotating field, so that the control of contacts sliding on a commutator, which is basically also conceivable but is susceptible to wear, is omitted. Such electronically controlled machines are also designated as brushless disc armature machines (GB 2 275 371 A). Furthermore, brushless disc armature machines are also known in which the rotor has two outer armature discs which extend radially to in front of the end faces of the coil cores which are fixed on the housing, the permanent magnets being retained in these armature discs with their pole faces aligned with the respective associated end faces of the coil cores (e.g. DE 197 01 797 A, FIG. 18; DE 42 14 483).
The object of the invention is to improve the efficiency of the known electric machines and to minimise any decline in the power due to reduction of the magnetisation of the permanent magnets over the course of time. At the same time the machines should be economical and simple to produce and should have a favourable unit weight.
Starting from an electric machine of the type mentioned in the introduction, this object is achieved according to the invention in that in each case pairs of legs, which succeed one another in the peripheral direction and are each provided with a pole face of different polarity on the free end facing the coil, of the permanent magnets provided in the two opposing outer armature discs are connected to one another in the end regions remote from the pole faces by a respective yoke which encloses the magnetic field and is made from soft or hard magnetic material, that the armature discs are connected to one another by radially extending walls which form the cavity between the armature discs into a plurality of chambers which are offset with respect to one another in the peripheral direction and are open towards the electromagnet components, and that the radially inner holes provided in the armature discs each open into the chambers of the rotor. The permanent magnets which are associated with the electromagnet components disposed centrally between both armature discs and thus are rigidly fixed on the peripheral wall of the housing of the machine are constructed in the manner of horseshoe magnets by means of the yokes which in each case connect two single magnets which succeed one another in the peripheral direction, resulting in a construction of the motor in which the magnetic field of the permanent magnets is kept contained as far as possible within the magnetic material of the permanent magnets, connected by the yoke, of the coil core, so that a low magnetic leakage field can be produced only in the narrow air gap between the end faces of the coil cores of the electromagnet components and the pole faces of the electromagnets. Losses of the magnetic rotating field generated in the electromagnetic components when the machine is switched on as motor are therefore minimised. This also applies when the machine is used as generator, which suggests itself for example for wind generators in which a direct coupling of the axis of rotation of the rotor to the output shaft of the wind wheel is possible without connection of a reduction gear between them. In addition to the favourable electric efficiency of the generator, the mechanical losses in the transmission gear are also avoided. In this case the radially extending walls which connect the armature discs act like the blades of a fan, i.e. via the radially inner holes in the armature discs air is drawn out of the housing and is blown radially outwards between the electromagnet components as a cooling air stream.
In the simplest case a row of electromagnet components is provided in the interior of the housing of the machine, and then the rotor has two outer armature discs guided on opposing sides in front of the coil core end faces of the electromagnet components.
The power of the machine can be increased without increasing the diameter in that two or more rows of electromagnet components spaced from one another in the longitudinal direction of the rotor shaft are disposed in the interior of the housing, wherein in addition to the two outer armature discs which are guided in front of the outer end faces of the coil cores, pointing in opposite directions, of the outermost rows the rotor has an additional armature disc with permanent magnets guided into each space between adjacent rows of electromagnet components in front of the end surfaces thereof which face one another, and the pole faces of differing polarity of the permanent magnets which are each exposed on opposing sides of the respective additional armature disc are aligned in the radial direction with the end faces of the coil cores of the rows of electromagnet components. Thus the permanent magnets disposed in the additional armature discs arexe2x80x94unlike the permanent magnets in the outer armature discsxe2x80x94constructed not as horseshoe magnets but as bar magnets of short length in order thus to keep the air gaps small between the pole faces of these magnets and the facing end surfaces of the coil cores of the electromagnet components and thus to avoid losses due to magnetic leakage fields.
In this case the interior of the housing is advantageously closed off and sealed against the external atmosphere, this sealing being achieved in the region of the shaft of the machine extending beyond the housing by a corresponding seal. The air drawn in from the housing through the radially inner holes in the armature discs and blown through between the electromagnet components takes up heat from the coils of the electromagnet components. Over the outer periphery of the armature discs the circulated air then passes into the space between the rotor and the end walls of the housing and is drawn back to the radially inner holes. In this way a self-contained air flow is produced in the interior of the housing and acts upon not only the electromagnet components but all other exposed surfaces in the interior of the housing and prevents the occurrence of locally higher temperatures (hot spots).
For removal of the heat produced in the machine and given off into the circulated air it is recommended that the outer and/or inner face of the housing be provided with ribs in order enlarge the surface of the housing which gives off or takes up heat. In this special case it may be advantageous if radially extending ribs are provided on the inner faces of the housing end walls facing the rotor and between these ribs radial channels are formed for the return of the gaseous atmosphere circulated in the interior of the housing.
Moreover the radial channels can be closed off on the armature disc side by a metal plate so that between the radial ribs channels are produced which are open only on the radially inner and radially outer end and are connected to the interior of the housing and through which the circulated air is returned.
The cooling of the electric machine according to the invention can also be carried out by through flows of ambient air, in that air inlet openings are provided in regions of the housing lying opposite the holes in the armature discs and air outlet openings which are offset radially outwards are provided in the housing.
In this case the electromagnet components are disposed at uniform spacings in the peripheral direction and protrude from the inner face of the peripheral housing wall into the space formed between the armature discs.
An embodiment in particular of the machine of the type mentioned in the introduction may be advantageous in which each pole face of the permanent magnets has in the peripheral direction an extent which covers two pole faces of the coils of two electromagnet components which succeed one another in the peripheral direction, and that the control means is designed so that in order to drive the rotor this control switches over the polarity of every second one of the electromagnet components which succeed one another in the peripheral direction with each rotation of the rotor about an angular spacing which corresponds to the angular spacing between two electromagnet components which succeed one another in the peripheral direction. This results in a 2-phase machine.
Alternatively three pole faces of the coils of three electromagnet components which succeed one another in the peripheral direction can be associated with each pole face of the permanent magnets, in which case the control means is designed so that in order to drive the rotor, after the rotor has rotated by an angular spacing corresponding to the angular spacing between electromagnet components which succeed one another in the peripheral direction, the control means switches over the polarity of every third one of the electromagnet components which succeed one another in the peripheral direction and thus a 3-phase machine is produced.
Basically more than three pole faces of the coils of electromagnet components which succeed one another in the peripheral direction can be associated with each pole face of the permanent magnets, in which the case the control means is designed so that in order to drive the rotor, after the rotor has rotated by an angular spacing corresponding to the angular spacing between electromagnet components which succeed one another in the peripheral direction, the control means successively switches over the polarity of every one of the electromagnet components, which succeed one another in the peripheral direction, of the group of electromagnet components associated with a permanent magnet.
By interconnection in each case of the coils of groups, which succeed one another in the peripheral direction, of electromagnet components which are associated in each case with a permanent magnet, the construction costs of the power electronics controlling the polarity of the coils relative to individual control of each individual coil are markedly reduced. Thus the electric control circuit in a 2-phase machine is also reduced to a 2-phase inverter control with variable amplitude and frequency. The four coils of four electromagnet components are located between two opposing horseshoe-shaped magnet segments. The first and the third coils are switched over simultaneously and in the next step the second and the fourth coils are switched over. The polarity of the first and third as well as of the second and fourth coil are opposed. The first and third as well as the second and fourth coil can therefore be firmly wired to one another, since the third and fourth coil are each controlled in mirror image to the first and second coil. Thus it is possible in practice to produce a 4-phase control with a 2-phase control.
The direction of rotation and the speed of the motor can be determined in that the switch-over point for the coils for switching over from one polarity to the other is controlled by way of a position pick-up which senses the relative rotational position of the rotor in the housing. Therefore the switch-over point for the control of the coil is determined by the sensor which predetermines the frequency. In this case the switch-over frequency is not identical to the motor speed, but it applies for one magnet segment. If the motor consists of ten segments with two switch pulses per phase and segment, then the switching frequency is 20 times the motor speed.
In this case a contactless sensor, particularly an optical sensor, which senses the relative rotational position of the motor with respect to the housing is provided as position pick-up.
The electromagnet components can each be advantageously held on separate support elements which can each be installed in an associated opening the in the peripheral wall of the housing in such a way that the pole faces of the coils of the electromagnet components are in the prescribed assembly position in alignment with the pole faces of the permanent magnets between the armature discs. In the event of damage to or failure of the coil of individual electromagnet components, these electromagnet components can then be removed individually and repaired or replaced by new electromagnet components which are capable of functioning.
Alternatively the pre-installed arrangement of the electromagnet components in an annular mounting, which in turn is held in the interior of the housing, is also conceivable. However, this construction presupposes the possibility of separate installation of at least one of the armature discs of the rotor.
The electromagnet components can each also have two separate coils with opposed directions of winding, i.e. reversed-polarity differential windings, which can then be selectively controlled by an electric or electronic control device. In this way the polarity of the respective electromagnet components is reversiblexe2x80x94depending upon the control of the respective coil winding.
If the machine according to the invention is to be used as a generator, the ends of the electrical conductors of each electromagnet component which form the coil winding are advantageously connected to the input connections of a separate rectifying circuit, and the rectifying circuits are then interconnected on the output side to a pair of electric bus lines. The direct current made up of the sum of the direct currents generated in the individual coils the respective associated rectifying circuit can then be tapped from the bus line. This embodiment is recommended particularly when the generator is driven at alternating speeds, such as may be the case for example with electric current generation using wind power in wind generators with rotor blades without blade adjustment due to differing wind speeds and wind strengths.
Then in an electronic inverter circuit which is connected downstream the direct current generated by such a generator can be converted into an alternating or three-phase current which is synchronised with the power supply.
The invention is explained in greater detail below in the following description of embodiments in conjunction with the drawings, in which: